Chrome oxide green is an important inorganic pigment. The oxide of trivalent chromium which crystallizes in the corundum latice has long been known. It is a pigment with high coloring and covering power which is thermally and mechanically stale up to high temperatures and inert towards most chemicals. Because of its deep green color, it takes up an important place among inorganic pigments.
Due to its chemical resistance, chrome oxide green also serves as starting material for the preparation of fire resistant materials used inter alia for lining melting furnaces or vats, e.g. for the manufacture of glass. Chrome oxide green is also used for surface finishing by the flame and plasma spraying process because of its great hardness. The trivalent oxide also serves as starting material for chromium carbide and metallic chromium.
Chrome oxide pigments are mainly obtained by the reduction of alkali metal chromates, starting with solid salts or aqueous solutions. The reducing agents used are elementary sulphur or compounds containing sulphur or carbon or compounds containing carbon. The reaction is carried out at normal pressure or at elevated pressure up to temperatures of about 1000.degree. C. After the solid reaction products have cooled, the formed soluble salts and the chromium (III) oxide are separated by mashing with water, filtration and washing. The filter cake is then dried and ground. The particle size and therefore color shade vary mainly in dependence upon the temperature (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, volume 18, page 605, 1979). When the primary particle diameter is relatively small, the color is a relatively light green with a yellow tinge whereas larger particle sizes result in darker green pigments with a blue tinge.
One disadvantage of the chrome oxide greens available on the market is that they require a very large quantity of water for mashing. This is liable to impair the strength of the colored products, for example when the pigments are used for coloring concrete, and when such a chrome oxide green is used in the manufacture of refractory products, the products show an undesirable formation of pores and excessive shrinkage when fired. These chrome oxide greens also tend to persistent tackiness and formation of lumps and therefore frequently give rise to difficulties in dosing and mixing processes.
It is an object of the present invention to provide an improved chrome oxide green which does not have the disadvantages described above.